Typically, a vehicle having a drive source includes various power transmission members, such as mating gears, a dog clutch including a dog teeth and a dog hole, a chain and a sprocket, and mating splines in the power transmission path from the drive source to a wheel. Each of these power transmission members engages with other adjacent power transmission members with a predetermined slack. Apparently, this slack does not exist during steady power transmitting condition. However, by changing a rotational speed of the drive source or applying an engine brake, a rotational speed difference occurs between a power transmission member upstream in the power transmission path and another adjacent power transmission member downstream in the power transmitting path. Thereby, contact surfaces of these power transmission members depart from each other temporarily within a range of the slack. The departed power transmission members, eventually, re-contact with each other, but this time, with contact surfaces on the opposite direction of the slack. If the re-contact speed and/or transmitting torque at the time of re-contact is relatively large, an operator/passenger of the vehicle may feel an unpleasant shock transmitted throughout the entire body of the vehicle.
To address the problem, for example, Utility Model Publication Unexamined No. H5-57363 discloses a method of reducing the transmitting torque between the power transmission members by controlling an ignition based on a throttle opening of an internal combustion engine which is the drive source. However, in such control based on the throttle opening, since a retard compensation is performed irrespective of a change in torque of the internal combustion engine, the retard compensation is not carried out at a suitable timing. Therefore, another problem in which an acceleration performance falls arises.
Accordingly, Patent Publication Unexamined No. 2003-65196 and Patent Publication Unexamined No. 2003-343408 disclose a method of reducing the transmitting torque between the power transmission members by calculating a degree of acceleration/deceleration of one of the power transmission members based on a rate of increase/decrease (derivative value) in the rotational speed of the power transmission member, and controlling an ignition based on the degree of acceleration/deceleration when the degree of acceleration/deceleration exceeds a predetermined value. However, such control does not operate when the degree of acceleration/deceleration is relatively small.